1. Field of the Invention
The present invention relates to a gain control circuit for a low-noise amplifier, specifically to a gain control circuit for a low-noise amplifier which can maintain linearity under input signal variations.
2. Description of the Related Art
In cellular mobile communication systems such as portable telephone systems, the strength of the signal varies depending on the distance between the cell site and the mobile phone, and also depending on the environment, hence a gain control circuit is required to overcome these factors. That is, if the input signal strength to the mobile phone becomes greater, a receiver circuit which comprises a low-noise amplifier, a mixer and an intermediate frequency amplifier becomes saturated, resulting the abrupt deterioration of linearity. Thus, a circuit which can reduce gain is required to prevent such deterioration of linearity. Also, as the signal strength becomes higher, linearity of the circuit deteriorates abruptly. Therefore, the gain control circuit which can improve linearity when a high strength signal is input is indispensable.
In the conventional gain control circuit, when a high strength signal is input, the gain control circuit attenuates the signal simply by a gain control voltage to decrease gain. Such gain control circuits can control gain by signal attenuation, but since they do not compensate harmonics they cannot improve the linearity of the low-noise amplifier.
FIG. 1 is a block diagram of the prior art low-noise amplifier having a gain control feature.
In a conventional low-noise amplifier, impedance matching is done at the input stage for noise matching. For this, a noise matching circuit 11 is provided at the input stage, and impedance matching circuits 12 and 13 are provided at the intermediate and output stages. A gain control circuit 14 is provided at the intermediate stage in order not to affect the noise and impedance matching at the input and output stages. When high strength signal is input, the gain control circuit attenuates the signal simply by a gain control voltage to reduce gain. In this case, gain control is possible by the signal attenuation, but since no compensation of harmonics exists, the linearity of the low-noise amplifier is not improved.
In general, the linearity of an amplifier becomes abruptly deteriorated when the circuit starts to be saturated by a high input signal. When two signals having identical signal strength and slightly different frequencies are applied, harmonics and intermodulation components are generated due to the non-linearity of the low-noise amplifier.
FIGS. 2a and 2b are graphs showing the input and output power characteristics of the harmonics spectrum and intermodulation components due to the non-linearity of the signal. Here, 2w.sub.1, 2w.sub.2, 3w.sub.1, 3w.sub.2 denote harmonics and w.sub.2 -w.sub.1, 2w.sub.1 -w.sub.2, 2w.sub.2 -w.sub.1, w.sub.1 +w.sub.2, 2w.sub.1 +w.sub.2, 2w.sub.2 +w.sub.1 denote intermodulaton components. We define IMD.sub.3 as the power difference between the basic component and the 3rd-order intermodulation component 2w.sub.1 -w.sub.2 or 2w.sub.2 -w.sub.1. Also, we define IP.sub.3 as the power which can be acquired by extrapolating the power of the basic component and the power of the 3rd-order intermodulation component. These two factors are used as measures of the linearity.
The 3rd-order intermodulation component is important for the linearity, because the 3rd-order intermodulation component is located in close proximity to the basic component so that it is difficult to remove it even with filters having very good selectivity, causing interference with the basic component.
When the low-noise amplifier is not at the fully saturated area and has weak distortion, it is possible to predict the linearity using Volterra series as shown in Equation 1.
Equation 1 ##EQU1##
Here, wd=w.sub.1 -w.sub.2. From equation 1, it can be noticed that IMD.sub.3 characteristics of the low-noise amplifier has a close relation with the magnitude of the harmonics. Hence, a compensation circuit for the harmonics is required to improve the linearity.